This invention relates to a holographic display element which includes a reflection hologram onto which light rays carrying images of display information are to be projected from a luminous image source. The holographic display element is useful, for example, as the combiner of a head-up display system in an automobile or as a display in a building window.
Holographic head-up display systems are already in practical use in aircraft cockpits. Recently efforts have been directed to the development of holographic head-up display systems for automobiles since holographic displays have various merits such as large freedom of layout, highness of wavelength selectivity and possibility of affording lens characteristics.
In most of hitherto developed or proposed head-up display systems for automobiles a reflection hologram formed on a transparent substrate is incorporated in the windshield, and light rays carrying images of display information are projected onto the hologram from a luminous display such as a cathode-ray tube positioned beneath the windshield. The driver or another observer on the vehicle can view the images of display information reflected by the hologram while viewing the forward outside real world through the windshield.
However, there is a problem about diffraction of external light such as sunlight by the hologram in the windshield. As external light such as sunlight is incident on the hologram from the outboard side of the windshield the hologram diffracts the incident light into various wavelengths of light at various angles. When wavelengths in the range from 560 nm (greenish yellow) to 780 nm (red) are diffracted into a certain range of angle with the road surface, the diffracted light is visible to persons viewing the windshield from the outside, such as pedestrians and drivers or passengers on cars running in the opposite direction, as a glaring and uncomfortable color and hence gives them a strange impression.